This invention relates to the discovery of a highly beneficial solvent for use in the bromination of a styrenic polymer.
Brominated styrenic polymers, e.g., brominated polystyrene, are well recognized flame retardants for use in engineering thermoplastics, e.g., nylon, polyethylene terephthalate, polybutylene terephthalate, etc. These flame retardants are prepared by reacting a brominating agent, e.g., bromine or bromine chloride, with a polystyrene in the presence of a Lewis acid catalyst. Since the styrenic polymer is usually very viscous or a solid, it has been deemed necessary to provide it to the reaction as a solute in a lower viscosity solution. The art generally describes the solvent as being a halogenated hydrocarbon, and, more specifically, a chlorinated hydrocarbon. Methylene chloride and dichloroethane are universally considered to be the solvents of choice. Despite their apparent attractiveness, these two solvents are not without disadvantage.
Methylene chloride is favored as it is relatively inert in the process. However, it is disadvantaged because it has a low boiling point and is challenged as being potentially depletive of the global ozone layer.
In distinction, dichloroethane has an acceptable boiling point and, in use, is more environmentally desirable than methylene chloride. It is not, however, a panacea as it reacts in the process to a significant extent and its use is not associated with an exceptionably low color brominated polystyrene product.
It is, therefore, an object of this invention to provide a solvent for use in the bromination of styrenic polymers which is environmentally beneficial, has a boiling point in the range of 80-95.degree. C., is relatively inert in the bromination process, and is associated with a near-white brominated styrenic polymer product.